At 0900, Vallery ordered the paravanes to be recovered. The Ulysses slowed down. The First Lieutenant, Lieutenant-Commander Carrington, went to the fo'c'sle to supervise operations: seamen, winch drivers, and the Subs, in charge of either side closed up to their respective stations.
Quickly, the recovery booms were freed from their angled crutches, just abaft the port and starboard lights, swung out and rigged with recovery wires. Immediately, the three-ton winches on 'B' gun-deck took the strain, smoothly, powerfully; the paravanes cleared the water.
A "boom", in this case, is a long pole. A recovery boom on a ship would be a long pole meant to recover something from the water, such as a small boat, a small submarine or, in this case, a paravane. A "crutch", in this case, would be a supporting structure where the boom is stored when not in use -- it sits in the crutches, which are probably U shaped or Y shaped metal structures.
The Wikipedia entry for Paravane (weapon) has a couple pictures of booms lowering a paravane into the water. Someone more familiar with the actual HMS Ulysses might have more detail on what theirs looked like but I imagine the images you see there are the same general concept.
So they are talking about releasing the metal pole from where it's stored and swinging it out so that they can latch onto the paravane and haul it up.